Pen type drug delivery devices have application where regular injection by persons without formal medical training occurs. This is increasingly common among patients having diabetes where self-treatment enables such patients to conduct effective management of their disease.
In certain types of medication delivery devices, such as pen type devices, cartridges of medication are used. These cartridges are housed in a cartridge holder or cartridge housing. Such cartridges include a bung or stopper at one end. At the other end of the cartridge, the cartridge may comprise a pierceable seal. To dispense a dose of medication from such a cartridge, the medication delivery device has a dose setting mechanism that uses a spindle to move in a distal direction towards the cartridge and to press a distal end of the spindle against the bung. This expels a certain set or preselected dose of medication from the cartridge. In order to insure dose accuracy, it is important that the distal end of the spindle remains on the bung of the cartridge before, during and after injection of a dose of medicament.
One perceived disadvantage of certain known medication delivery devices is that because of the various tolerance differences that may occur during manufacturing (e.g., tolerance differences that may arise during component molding) of the various parts making up the drug delivery device and the desire to not pre-load the bung axially in the assembled device, there may be a gap between the end of the spindle and the cartridge bung after the medication delivery device has been assembled. In other words, when initially assembled, the cartridge (and hence cartridge bung) may not be in contact with the distal end of the spindle. Therefore, if a user using the drug delivery device for the first time dials a dose, the actual dose received may be equal to the dialed dose less the initial gap between the distal end of the spindle and cartridge bung. The air gap between the cartridge bung and distal end of the spindle may be equivalent to a dose that causes the received dose that is outside preferred dose accuracy limits. For example, this air gap may be equivalent to the loss of between 0 and 10 units (i.e., 0-0.14 milliliters) of drug product on the first dose.
There is, therefore, a general need to take these perceived issues into consideration when designing either resettable or non-resettable drug delivery devices, such as pen type drug delivery devices.